Can't use Radeon GPU to render in cycles (R9 270)
R9 270 is actually GCN 1.0 architecture and is not supported after Blender 2.78c.

If you look at the wikipedia chart for the Radeon 200 series products, you'll see that the 270 is actually a GCN 1.0 hardware. Which was dropped from Cycles in Blender 2.78c.